The present invention relates to a distance-measuring device according to the preamble of claim 1 or 2.
Conventional distance-measuring devices preferably operate in the near range using inductive, capacitive or ultrasonic sensors. For a measurement with inductive sensors, the calibration curve must be established and also the material of an object to be measured must be known. Furthermore, the inductive sensors have a measuring range of, for example, 180xc2x0, so that two sensors located next to each other mutually influence each other and thus the calibration curves of the respective sensors can vary. Moreover, such sensors are available commercially only in embodiments that have a diameter greater than 4 mm (M4).
The disadvantage of a measurement with capacitive sensors is that the distance between the capacitor plates must be known exactly. Furthermore, the measurement is subject to influence by atmospheric humidity, general electromagnetic compatibilities or temperature. In order to be able to perform the measurement independently of those parameters it is necessary, depending on the requirement, to perform a reference measurement by means of which the interfering influence can then be eliminated.
Further known from U.S. Pat. No. 3,522,527 are two cavity resonators with which the distance to corresponding surfaces is measured, the distance and thus the thickness between the two surfaces being determined indirectly by placing the two cavity resonators opposite each other. To perform this measurement, each of the cavity resonators must have a separate sensor, which conventionally is connected to the cavity resonator in a complicated manner and hence is associated with a correspondingly large expense for equipment.
Hence the problem addressed by the present invention is to create a distance-measuring device for determining the distance which overcomes the above-cited disadvantages and allows a continuous determination of distance, easy handling and diverse possible uses.
That problem is solved with the device features of claim 1 or 2.
According to the invention, the sensor has a resonator with a coplanar slot coupling, and specifically in the form of a cavity resonator. With this measure the advantage is achieved that extremely small embodiments, for example  less than M4, are realizable and the possible uses are increased by a multiple. Owing to the basic geometry of a cavity resonator, small distances between several parallel sensors are possible, because the sensor has a laterally sharply limited measuring range and thus its measuring behavior is not influenced by parallel sensors. As a field of application it is conceivable that the distance-measuring device according to the invention could be used to detect the direction of moving objects or for a space-saving configuration, e.g., by means of parallel configuration.
The sensor according to the invention can also be used as a switch with which changes of the switching point are possible without any redimensioning or modification of the sensor element or addition of other electronic components. That achieves the advantage that the switching point can be adjusted to the specific requirements via software, for example.
The sensor according to the invention is also able to detect approaching conductive or dielectric objects and to measure the distance to the object within the micron range. This type of sensor can be used, for example, as a proximity switch for continuous measurement of the piston travel at the reversal point of pneumatic and hydraulic cylinders, of valve positions or for measurement of the extension of pressure membranes.
According to the invention, the measuring distance for conductive objects does not depend on the object""s size if it is assumed that the object is at least as large as the diameter of the cavity resonator. Moreover, a measurement of distance to conductive and dielectric objects is generally possible.
If the sensor is used as a switch, then according to the invention a change of the switching point or a redimensioning or modification of the sensor element can be implemented in a simple manner. Since the switching point is adjustable via software, for example, there is the further advantage that multiple switching points can be input in a simple manner via suitable software, whereby one obtains a substantially more versatile range of uses, e.g., for detecting the sizes of parts, for different configurations of a machine, for detecting rotation angles via cams, etc. In contrast, as mentioned initially, very great effort is required to implement multiple switching points with inductive sensors.
Owing to the measurement method used in the distance-measuring device according to the invention, several switching points can also be connected to one another via a logic circuit, whereupon the measurement method operates continuously. For example, this is advantageous if three switching points are needed for the interrogation of a rotary cylinder.
Owing to its compact construction, one base element is usable in all standard housing types for switching distances of, for example, 0.6, 0.8, 1.0, 1.5, 2.0 or 5 mm, resulting in cost savings and hence reduced logistic requirements.
Alternatively, the distance-measuring device, specifically the resonator, can have a microstrip line for the in-coupling, which is used especially when it is advantageous for the evaluation electronics unit to be offset from the resonator, e.g., for applications in which a high temperature occurs.
Other advantageous embodiments are the subject of other subclaims.
It has turned out to be especially advantageous if the resonator is a radio frequency resonator whose resonance frequency lies between 1 and 100 GHz depending on the object, and preferably between 20 and 30 GHz. For certain applications it is further advantageous to tune the radio frequency resonator with a frequency between 22 and 24 GHz as well as 24 and 26 GHz or any other range, with a bandwidth of preferably 2 GHz or with a bandwidth of approximately 10 percent of the utilized frequency.
If the distance-measuring device according to the invention is equipped with a resonator which has a cylindrical shape and whose base surface facing toward the object is open, i.e., not metallized, then the resonance frequency is not dependent on temperature.
If the cavity resonator according to claim 5 is filled, for example, with a dielectric, preferably Al2O3, then the entire distance-measuring device can be small.
Here it should be pointed out that it is generally advantageous if the measuring range is as large as possible, but that means that the dielectric constant xcex5 should be small. Ideally, that is achieved in that the cavity resonator is unfilled, i.e., contains no dielectric. But a disadvantage of that arrangement is that the cavity resonator then has to be large in order to obtain a large measuring range. But with dielectric the cavity resonator is small for approximatelythe same measuring range. However, it must be made certain that the dielectric constant of the dielectric is not too large (preferably xe2x89xa610), since otherwise the losses increase and the range of distances decreases. If a ceramic is used as dielectric, the further advantage is achieved that applications requiring resistance to temperatures of up to 1000xc2x0 C. are possible and use for highly dynamic measurements of pressure in internal-combustion engines is possible. Thus the distance device according to the invention is resistant to pressure and hence also usable in hydraulic cylinders, for example.
It has proven advantageous that, according to claim 8, only the surface of the dielectricxe2x80x94with the exception of the base surface facing toward the objectxe2x80x94is coated or sputter-coated with a thin layer of gold, so that the temperature function depends only on the temperature coefficient of the ceramic, for example, and not on the housing.
The sensor element consists of a ceramic and a metal housing and can be connected to the evaluation electronics unit via a suitable radiofrequency line, e.g., a waveguide. Because of that, it is possible to use the sensor element for high-temperature applications at up to approximately 1000xc2x0 C., e.g., in internal combustion engines.
Independently of the measurement of a distance, the distance-measuring device can also be used advantageously for the measurement of other physical quantities such as pressure, force or mass and of material properties such as the loss factor of dielectric materials. For that purpose, the open side of the cavity resonator is closed with a sample of the material at a fixed distance to it. For a pressure sensor, preferably a piezoelectric ceramic disk would be mounted at distance zero. If a pressure, a force or a mass now acts on the piezoelectric ceramic, then the latter""s dielectric constant changes. The change of the dielectric constant results in a shift of the resonance frequency. By determining the resonance frequency with the device features from claim 1 or 2, the pressure, force or mass on the piezoelectric ceramic can be determined.
If, according to claim 10, the dielectric is inserted into a metal housing made preferably of Kovar or titanium, a suitable high-temperature application is conceivable. Then the cavity resonator in the unfilled state has a high measuring accuracy even at high temperatures, and in the filled state the expansion as such is exactly controllable.
If the distance-measuring device according to claim 11, and specifically the resonator, has a coplanar slot coupling on the side facing away from the object, that arrangement ensures that the in-coupling of the resonance frequency can occur simply and at a suitable point.
Depending on the operating mode of the distance-measuring device, the coplanar slot coupling can consist of one coupling slot each for the transmitter and receiver according to claim 12, which are disposed circularly and which corresponds to a transmission mode, or the coplanar slot coupling consists of one coupling slot for the transmitter and receiver, which corresponds to operation in a reflection mode.
If, according to claim 14, the distance-measuring device is operated in the H0np mode, preferably in the H011 mode, then the resonator can oscillate within a large range of resonance frequencies in which no other modes are co-excited, so as to keep the measuring accuracy high. Furthermore, excitation of the H011 mode offers the advantage that then no wall currents flow over the edges between the cylindrical surface and the end surface.
Other advantageous embodiments of the invention are the subject of the other subclaims.